The present invention relates to an improved thermal energy storage material intended, among other uses, to provide an efficient phase change or eutectic material for use in storing thermal energy. Phase change or eutectic materials have shown potential for use as efficient heat storage systems, however, persistent technological and economic problems in the containment and use of the eutectic materials for heat storage have prevented the commercial adaptation of these materials to thermal energy storage. The phase change material of the present invention is designed to be brought into direct contact with hot or cool exchange fluids in a thermal energy storage system and eliminate the need for providing an elaborate containment system for the phase change material.
The industrial community has long been involved with the practice of heat storage for the purpose of conserving or transferring sensible heat energy without the use of a phase change. For example, steelmaking open-hearth furnaces are coupled with thermally insulated checkerworks of ceramic bricks which alternately are heated by hot combustion product gases and cooled by the air used in the combustion process. In this manner the sensible heat of the waste gases is used to preheat the combustion air. Such heat storage principles also have residential applications; many homes are heated at night by stored heat energy generated by the daytime sunshine.
The electric utility industry is currently in search of a method to efficiently store of heat for the purposes of "load-leveling"; that is, the efficient operation of power plants (nuclear, solar, coal- or oil-burning) at a constant rate around the clock such that the power plants store heat, or electrical energy, generated at night for use during the peak demand daytime hours. Improved efficiency in the 24-hour operation would provide a way to satisfy the peak demand without necessitating the construction of expensive new utility power plants. For instance, in the case of solar power plants the availability of heat from solar radiation can be extended through a period of darkness by an efficient phase change heat storage material.
Further, in various industrial plants involved with the production, processing or fabrication of metals, ceramics or glass, etc., at elevated temperatures, a need exists to minimize the heat lost from the hot process gases. Such plants design processes to transfer the heat in hot process gases to a cool medium requiring heating, e.g., the air needed for the combustion system. A heat storage intermediary material such as the hypereutectic material of the present invention, has the potential of direct applicability to these processes.
Current state-of-the-art high temperature heat energy storage commonly involves the cyclical process of melting a eutectic material having a high latent heat-of-fusion during the storage half of the cycle and liberating the latent heat-of-fusion upon solidification of the eutectic material during the utilization half cycle. Problems are encountered when the eutectic material experiences such cyclic melting and solidification. The volume change associated with the cyclic phase change often causes damage to the eutectic material or its containment structure. In addition, the eutectic material is often corrosive to the heat exchanger material, as are the hot gases, fluids, or condensates resulting from various combustion processes or heat sources. Thus, containment of the eutectic material in a heat exchanger insensitive to corrosion and unaffected by volume changes generally represents a difficult and expensive problem in the design of phase change heat storage systems which has not yet been solved.
Previous investigations have considered the storage of heat energy using the latent heat-of-fusion of particular materials. Generally, in such concepts, the eutectic material would be contained in corrosion-resistant tubes. The entire eutectic material within the tube would undergo fusion and freezing upon each heat storage cycle. In such a system, the phase change material is contained within the tubes and is isolated from direct contact with the working fluids. In some prior art systems the eutectic material directly contacts the working fluids. For example, U.S. Pat. No. 4,192,144 shows the use of pebbles as a direct contact heat storage medium. The pebbles comprise nonmetallic shells of, for example, silica, alumina or graphite, which cover or enclose a eutectic salt material. Other prior art systems, for instance, U.S. Pat. No. 4,421,661, show a eutectic material which is retained within a porous solid support-structure material which itself is capable of storing only sensible heat.
The heat storage materials often accepted for use in such heat storage systems are eutectic compositions of multi-component fused salts, such as nitrates, carbonates, and halides. These materials may cause severe corrosion damage to the heat exchanger structure. In order to overcome the drawbacks normally associated with the use of salt eutectic materials, Birchenall and Riechman, Metall. Trans., 11A 1415 (1980) surveyed the heat storage potential of metallic eutectic materials, clarified the thermodynamic factors important to their storage of heat, and measured the heat storage capacity for a number of high-temperature binary and ternary metallic eutectic alloys. Several eutectic alloys showed promise: the binary Al-Si eutectic alloy (570 J/gm at 577.degree. C. (850 K.) for 12.6 wt. percent Si) and the ternary Al-Si-Mg eutectic alloy (550 J/gm at 506.degree. C. (779 K.) for 13.3 wt. percent Si, 4.6 wt. percent Mg). These alloy eutectic materials could be favored over eutectic salt materials, with the proper methods of containment and utilization. The alloy eutectics have a large range of transformation temperatures, comparatively low vapor pressures, and exhibit a high thermal conductivity. Therefore, less time or less heat exchanger surface would be required to melt or solidify the alloy eutectic materials.
However, the commercial containment of these eutectic phase change materials would require a heat exchanger having expensive corrosion resistant tubing. Currently, the development of high-temperature heat storage systems incorporating metallic eutectic material has been stifled by the lack of economic, efficient containment devices.
Therefore, there is currently a need for the selection and design of a system for storing thermal energy that utilizes either salt or alloy phase change materials that can withstand direct contact with hot, corrosive environments.
There is a further need for the development of an improved material and method for storing thermal energy which take advantage of the desirable characteristics of the eutectic alloy materials.
The object of this invention is to provide a simple and inexpensive self-contained, direct contact phase change heat storage material.
It is a further object of the invention to provide a phase change material having generally a spherical shape that can be maintained in a container such that the phase change material can be brought into direct contact with the hot corrosive fluids in, for example, a ceramic lined tower, without the need for expensive and difficult containment.
In addition, it is an object of the present invention to provide a heat storage shot having a unique gross composition and processes for producing such shot so that a relatively high melting, corrosion resistant outer shell encloses and contains a lower melting, heat storing eutectic core of an optimized composition.